(Yes, but.)

Over the past few years, I’ve seen many articles about mysterious rise in colorectal cancer (CRC) in young people. There are various stories for why this might be happening:

General health. Maybe modern people are unhealthy (obesity, low physical activity, diabetes, poor sleep), leading to insulin resistance and chronic inflammation, meaning faster epithelial cell proliferation and a miscalibrated immune system that fails to stop early cancers?

Ultra-processed food. Maybe people are eating more ultra-processed foods that contain additives (like emulsifiers) that degrade colon mucus, allowing bacteria to contact epithelial cells and drive inflammation? Or maybe ultra-processed food has low fiber and glycemic load, leading to insulin resistance and chronic inflammation, with the problems mentioned above?

Bad meat. Maybe people are eating more red and/or processed meats, which expose the colon to nitrites and secondary bile acids, which inflame the epithelium and promote chronic inflammation?

The microbiome. Maybe it’s the microbiome. For example, maybe people’s guts are getting colonized by strains of E. coli that produce genotoxic colibactin. Or maybe overuse of antibiotics in early life depletes protective bacteria in the gut, allowing harmful strains to expand, e.g. strains of B. fragilis that cause inflammation, or strains of F. nucleatum that can survive in the gut and drive tumor growth?

Environmental exposures. Maybe people are getting exposed to bad stuff in the environment (microplastics, forever chemicals, pesticides, endocrine disruptors, air pollution) that does bad stuff (damages gut barrier, screws up the microbiome, disrupts hormonal signaling)?

Maternal health. Maybe poor maternal health (obesity, diabetes) exposes the fetus to elevated glucose / insulin / inflammation, and these in turn program the child for a lifetime of metabolic issues and inflammation?

Whatever. Maybe alcohol / smoking / painkillers / calcium / vitamin D / inflammatory bowel disease / hereditary syndromes / screening bias?

None of the experts seem to agree on which of these is the culprit, so I figured that I (person with blog) should help.

If you poke at these stories, most of them are individually pretty weak. It can’t all be detection bias since CRC deaths are also going up in younger people. And several proposed causes (air pollution, tobacco) have actually fallen in rich countries. Other explanation, like E. coli producing colibactin, seem biologically real, but there’s no evidence that they’re increasing over time. Still other suggested causes (microplastics, forever chemicals) are mostly mechanistic speculation at this point. Obesity, inactivity, and chronic inflammation also all seem biologically real, and they are likely increasing, but why should they specifically cause colorectal cancer in young people?

A plausible answer to that last question is that they aren’t. They’re doing it, but not specifically.

“Young people”

This will sound pedantic, but bear with me: If you say that CRC is increasing in younger people, what exactly does that mean? After all, the set of people who qualify as young changes over time. (Ever notice that you keep getting older?)

Siegel et al. (2026) plot how often CRC was found in different age groups in 1995 and in 2022.

They also provide this plot of how common different types of CRC are in different age groups.

At a glance, this doesn’t look so bad. If you’re young, you might think, “OK, my current risk is higher than previous generations faced at the same age, but I can look forward to decreasing rates when I’m old.” You could easily think this is good news: While there’s a relative increase when you’re young, it’s tiny compared to the absolute decrease while you’re old.

Unfortunately that’s the wrong way to think about it.

Downham et al. (2026) plot CRC rates in different age groups across the Anglosphere over time.

Everyone I’ve shown this plot to has said it’s confusing, so let me explain: The different lines track age-bands as people born in different years move in and out of those bands. For example, in the US plot in the bottom right, the “20-25” line starts with the left-most dot showing the CRC rate for people born between 1965 and 1970 when they were 20 to 24 years old (around 1990). The next dot shows the rate for people born between 1970 and 1975 when they were 20 to 24 years old (around 1995), and so on.

That figure is weird, because the lines connect different groups of people. I wanted a plot where there are lines for different birth cohorts as they age. For unknown reasons, no one seems to make such plots, and the data isn’t trivial to access. So I used a plot digitizer to click on every damned point that US figure above and then replotted it:

Now the individual lines show specific groups of people tracked through time. For example, the “1932.5” line shows CRC rates for people born between 1930 and 1935, when those people were at different ages. If you look closely, you’ll notice that these rates are higher those for people born between 1940 and 1945 for all ages (where we have data).

That was the pattern for a long time: Between 1920 and 1950, later generations enjoyed lower CRC rates across all phases of their lives. But between 1950 and 1960, that pattern reversed and since then later generations have had higher CRC rates at all ages.

We don’t know for sure what will happen in the future. But I think it’s likely this trend will continue. Yes, if you are currently young, you face higher CRC risk than previous generations did when they were young. That’s the bad news. The other bad news is that when you are old, you may also face higher CRC risk than previous generations did when they were old.

“Colorectal cancer”

The other other bad news is that CRC isn’t the only type of cancer that’s rising in later generations. Sung et al. (2019) give this plot:

These are again the confusing graphs where individual lines show age bands as different people move in and out of them. But you get the point: Lots of cancers are going up in younger people later generations, including uterine, gallbladder, kidney, liver, pancreas, and thyroid. (Their additional material contains plots for 18 other cancers.)

Note that these plots have a logarithmic y-axis, meaning the changes are larger than they might appear. Moving up a quarter of the way between two vertical ticks corresponds to an increase of a factor of ≈ 1.78.

If lots of cancers are becoming more common in later generations, then why is everyone talking about CRC? I think that’s because CRC in unique in that it is:

1. common
2. dangerous
3. increasing in later generations
4. treatable if caught early
5. detectable via screening

For example, thyroid cancer diagnoses have skyrocketed in recent decades. But that’s partly because of more detection, and thyroid cancer is highly treatable, without clear benefits from early detection. Pancreatic cancer also seems to be increasing, but we don’t have good ways to screen for it and even if we did, we don’t have good ways to treat it.

CRC is really unique in that you can save lives by telling people, “Hey! CRC is going up! You should get screened!” If you’re interested in public health, that’s the most important thing. But if you’re interested in unraveling the mystery of CRC going up, it’s important to note that CRC isn’t really unique at all.

TLDR

No:

Colorectal cancer is going up in young people.

Yes:

Various kinds of cancer are going up in later generations. (Definitely at younger ages, possibly at all ages.)

Reminder

This blog endorses colorectal cancer screening. We don’t yet know if colonoscopies are better than other methods of screening (sigmoidoscopy, stool tests), but we do know that screening is better than not screening. When caught early, CRC is highly treatable, often with only surgery (no chemotherapy or radiation) and a return to normal activities within a couple weeks.